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213 lines
8.5 KiB
HTML
213 lines
8.5 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2018 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being "Funding Free Software", the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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"GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Nested Functions (Using the GNU Compiler Collection (GCC))</title>
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<meta name="description" content="Nested Functions (Using the GNU Compiler Collection (GCC))">
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<meta name="keywords" content="Nested Functions (Using the GNU Compiler Collection (GCC))">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="C-Extensions.html#C-Extensions" rel="up" title="C Extensions">
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<link href="Constructing-Calls.html#Constructing-Calls" rel="next" title="Constructing Calls">
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<link href="Labels-as-Values.html#Labels-as-Values" rel="prev" title="Labels as Values">
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<style type="text/css">
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<!--
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a.summary-letter {text-decoration: none}
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blockquote.indentedblock {margin-right: 0em}
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blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
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blockquote.smallquotation {font-size: smaller}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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div.lisp {margin-left: 3.2em}
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div.smalldisplay {margin-left: 3.2em}
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div.smallexample {margin-left: 3.2em}
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div.smalllisp {margin-left: 3.2em}
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kbd {font-style: oblique}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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pre.menu-comment {font-family: serif}
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pre.menu-preformatted {font-family: serif}
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pre.smalldisplay {font-family: inherit; font-size: smaller}
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pre.smallexample {font-size: smaller}
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pre.smallformat {font-family: inherit; font-size: smaller}
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pre.smalllisp {font-size: smaller}
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span.nolinebreak {white-space: nowrap}
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span.roman {font-family: initial; font-weight: normal}
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span.sansserif {font-family: sans-serif; font-weight: normal}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Nested-Functions"></a>
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<div class="header">
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<p>
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Next: <a href="Constructing-Calls.html#Constructing-Calls" accesskey="n" rel="next">Constructing Calls</a>, Previous: <a href="Labels-as-Values.html#Labels-as-Values" accesskey="p" rel="prev">Labels as Values</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Nested-Functions-1"></a>
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<h3 class="section">6.4 Nested Functions</h3>
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<a name="index-nested-functions"></a>
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<a name="index-downward-funargs"></a>
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<a name="index-thunks"></a>
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<p>A <em>nested function</em> is a function defined inside another function.
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Nested functions are supported as an extension in GNU C, but are not
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supported by GNU C++.
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</p>
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<p>The nested function’s name is local to the block where it is defined.
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For example, here we define a nested function named <code>square</code>, and
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call it twice:
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</p>
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<div class="smallexample">
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<pre class="smallexample">foo (double a, double b)
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{
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double square (double z) { return z * z; }
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return square (a) + square (b);
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}
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</pre></div>
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<p>The nested function can access all the variables of the containing
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function that are visible at the point of its definition. This is
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called <em>lexical scoping</em>. For example, here we show a nested
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function which uses an inherited variable named <code>offset</code>:
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</p>
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<div class="smallexample">
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<pre class="smallexample">bar (int *array, int offset, int size)
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{
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int access (int *array, int index)
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{ return array[index + offset]; }
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int i;
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/* <span class="roman">…</span> */
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for (i = 0; i < size; i++)
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/* <span class="roman">…</span> */ access (array, i) /* <span class="roman">…</span> */
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}
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</pre></div>
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<p>Nested function definitions are permitted within functions in the places
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where variable definitions are allowed; that is, in any block, mixed
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with the other declarations and statements in the block.
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</p>
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<p>It is possible to call the nested function from outside the scope of its
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name by storing its address or passing the address to another function:
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</p>
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<div class="smallexample">
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<pre class="smallexample">hack (int *array, int size)
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{
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void store (int index, int value)
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{ array[index] = value; }
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intermediate (store, size);
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}
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</pre></div>
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<p>Here, the function <code>intermediate</code> receives the address of
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<code>store</code> as an argument. If <code>intermediate</code> calls <code>store</code>,
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the arguments given to <code>store</code> are used to store into <code>array</code>.
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But this technique works only so long as the containing function
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(<code>hack</code>, in this example) does not exit.
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</p>
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<p>If you try to call the nested function through its address after the
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containing function exits, all hell breaks loose. If you try
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to call it after a containing scope level exits, and if it refers
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to some of the variables that are no longer in scope, you may be lucky,
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but it’s not wise to take the risk. If, however, the nested function
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does not refer to anything that has gone out of scope, you should be
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safe.
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</p>
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<p>GCC implements taking the address of a nested function using a technique
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called <em>trampolines</em>. This technique was described in
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<cite>Lexical Closures for C++</cite> (Thomas M. Breuel, USENIX
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C++ Conference Proceedings, October 17-21, 1988).
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</p>
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<p>A nested function can jump to a label inherited from a containing
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function, provided the label is explicitly declared in the containing
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function (see <a href="Local-Labels.html#Local-Labels">Local Labels</a>). Such a jump returns instantly to the
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containing function, exiting the nested function that did the
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<code>goto</code> and any intermediate functions as well. Here is an example:
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</p>
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<div class="smallexample">
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<pre class="smallexample">bar (int *array, int offset, int size)
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{
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__label__ failure;
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int access (int *array, int index)
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{
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if (index > size)
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goto failure;
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return array[index + offset];
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}
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int i;
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/* <span class="roman">…</span> */
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for (i = 0; i < size; i++)
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/* <span class="roman">…</span> */ access (array, i) /* <span class="roman">…</span> */
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/* <span class="roman">…</span> */
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return 0;
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/* <span class="roman">Control comes here from <code>access</code>
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if it detects an error.</span> */
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failure:
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return -1;
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}
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</pre></div>
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<p>A nested function always has no linkage. Declaring one with
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<code>extern</code> or <code>static</code> is erroneous. If you need to declare the nested function
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before its definition, use <code>auto</code> (which is otherwise meaningless
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for function declarations).
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</p>
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<div class="smallexample">
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<pre class="smallexample">bar (int *array, int offset, int size)
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{
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__label__ failure;
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auto int access (int *, int);
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/* <span class="roman">…</span> */
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int access (int *array, int index)
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{
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if (index > size)
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goto failure;
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return array[index + offset];
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}
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/* <span class="roman">…</span> */
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}
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</pre></div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Constructing-Calls.html#Constructing-Calls" accesskey="n" rel="next">Constructing Calls</a>, Previous: <a href="Labels-as-Values.html#Labels-as-Values" accesskey="p" rel="prev">Labels as Values</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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